Hydrostatic transmissions have many uses, including the propelling of vehicles, such as mowing machines, and offer a stepless control of the machine's speed. A typical hydrostatic transmission system includes a variable displacement main hydraulic pump connected in a closed hydraulic circuit with a fixed displacement hydraulic motor. The closed hydraulic circuit includes a first conduit connecting the main pump outlet with the motor inlet and a second conduit connecting the motor outlet with a pump inlet. Either of these conduits may be the high pressure line depending upon the direction of pump displacement from neutral. For most applications, the pump is driven by a prime mover, such as an internal combustion engine or an electrical motor, at a certain speed in a certain direction. Changing the displacement of the main pump will change its output flow rate, which controls the speed of the motor. Pump outflow can be reversed, thus reversing the direction of the motor. In a vehicle, the motor is typically connected through suitable gearing to the vehicle's wheels or tracks.
In some vehicles, such as zero-turn-radius mowers, separate hydraulic pumps and motors are used to independently drive separate wheels of an axle. By independently driving the wheels in opposite directions, for example, the vehicle can be made to turn with zero radius. Zero-turn-radius mowers are increasingly popular as the size and costs of such mowers decrease.
Movement of some zero-turn-radius mowers is controlled by a pair of steering arms connected to each hydraulic pump. By moving the steering arms, an operator can control the pump flow displacement and direction of each pump and, thus, the speed and direction of rotation of each rear wheel. Accordingly, the operator can both steer the mower and control the speed of the mower by manipulating the steering control arms.
Mowers often are equipped with parking brakes that can be activated by an operator to secure the wheels against rotation when the vehicle is parked. Such parking brakes are typically normally disengaged, and are engaged upon activation by the operator. Thus, if the operator forgets to activate the parking brake, the mower could potentially roll if on uneven ground.